Rotary wheel cutters, in contrast to reciprocating blade cutters, have unique characteristics which are suitable for cutting plies of sheet material that are collectively relatively thin, for example, less than 1/4 inch thickness. The cutting action produced by a wheel comes about through a severance of the material when the sharp peripheral cutting edge of the wheel is brought into engagement with a support surface with the material therebetween. The edge severs the materials or fibers in what is believed to be both a crushing and cutting operation. A unique and advantageous characteristic of the rotary wheel cutting process is that there is basically no inherent limitation on the speed at which the severance of material takes place, nor upon the rate at which the cutting wheel operates producing that severing process. Consequently, a cutting wheel is a desirable tool for cutting a single ply of selected fabric material, for example, a suit.
One of the principle difficulties encountered in cutting single plies of sheet material is the retention of the sheet material in a fixed position throughout the cutting process. In systems employing an automatically controlled machine that operates from a predetermined program, the material cannot shift in the course of a cutting operation, otherwise, the pattern pieces that are cut will not conform to the program lines of cut. Also, since high speed is one of the main advantages of the cutting wheel, the machine should be designed to perform at high speed with minimum inertia and extra motion.
Vacuum hold down devices of the type which produce sub atmospheric pressure at a work or bearing surface on which the sheet material is spread have gained acceptance in the material cutting art. A vacuum hold down device is disclosed in U.S. Pat. No. 4,444,078. The vacuum hold down device includes a cutting wheel which rolls in cutting engagement with a bearing surface which has an array of openings therethrough communicating through channels with a vacuum pump. However, problems may be encountered when such apparatus is used to cut materials such woven fabrics. As the cutting instrument passes over the apertures in the bearing surface, threads which comprise the fabric may be forced into the apertures by the cutting instrument rather than being cut by it. As a result, pattern pieces cut from the fabric are not easily separated from the waste material. Further, failure to effectively sheer of all the threads which comprise the fabric in the area of an aperture may result in cut pattern pieces with rough or ragged edges.
U.S. Pat. No. 4,444,078 represents an approach of the prior art to solve the present problem. In the '078 apparatus, each aperture in the support surface includes a valve movable between an open position spaced from the support surface and closed position flush with the adjacent support surface. A support surface having a multitude of these valves and associated mechanisms for selectively activating the valves is extremely complex and hence expensive. Further, the large number of moving parts increases maintenance requirements.
Therefore, the need exists for a cutting system which cooperates with an apertured vacuum support surface to substantially separate the sheet material along a predetermined line. The need further exists for a method of cutting sheet material on an apertured vacuum support surface which reduces bridges of material between a desired part periphery and the remaining portion of the sheet. The need further exists for a cutting system that can be employed in existing vacuum systems for improving the performance of the system.